diff --git a/app/code/Magento/QuoteGraphQl/Test/Unit/Model/Cart/ValidateAddressFromSchemaTest.php b/app/code/Magento/QuoteGraphQl/Test/Unit/Model/Cart/ValidateAddressFromSchemaTest.php
new file mode 100644
index 000000000000..6ea36db739bb
--- /dev/null
+++ b/app/code/Magento/QuoteGraphQl/Test/Unit/Model/Cart/ValidateAddressFromSchemaTest.php
@@ -0,0 +1,137 @@
+<?php
+/**
+ * Copyright 2026 Adobe
+ * All Rights Reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\QuoteGraphQl\Test\Unit\Model\Cart;
+
+use GraphQL\Type\Definition\InputObjectField;
+use GraphQL\Type\Definition\StringType;
+use Magento\Framework\GraphQl\Schema\Type\NonNull;
+use Magento\Framework\GraphQl\Schema\Type\TypeRegistry;
+use Magento\Framework\GraphQl\Schema\Type\Input\InputObjectType;
+use Magento\QuoteGraphQl\Model\Cart\ValidateAddressFromSchema;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @see ValidateAddressFromSchema
+ */
+class ValidateAddressFromSchemaTest extends TestCase
+{
+    /** @var ValidateAddressFromSchema */
+    private ValidateAddressFromSchema $model;
+
+    /** @var TypeRegistry|MockObject */
+    private TypeRegistry $typeRegistry;
+
+    /** @var InputObjectType|MockObject */
+    private InputObjectType $cartAddressInputType;
+
+    protected function setUp(): void
+    {
+        $this->typeRegistry = $this->createMock(TypeRegistry::class);
+        $this->cartAddressInputType = $this->createMock(InputObjectType::class);
+        $this->typeRegistry->method('get')
+            ->with('CartAddressInput')
+            ->willReturn($this->cartAddressInputType);
+        $this->model = new ValidateAddressFromSchema($this->typeRegistry);
+    }
+
+    /**
+     * Address missing a nullable field passes validation.
+     *
+     * Covers the regression from ACP2E-4223 where CartAddressInput.telephone was changed
+     * to String! (NonNull), forcing telephone even when the store configures it as Optional.
+     * After the fix telephone is String (nullable) and this test must pass.
+     */
+    public function testAddressWithoutNullableFieldPassesValidation(): void
+    {
+        $firstnameField = $this->makeField('firstname', true);
+        $telephoneField = $this->makeField('telephone', false);
+
+        $this->cartAddressInputType->method('getFields')
+            ->willReturn(['firstname' => $firstnameField, 'telephone' => $telephoneField]);
+
+        // telephone absent entirely
+        $this->assertTrue($this->model->execute(['firstname' => 'John']));
+
+        // telephone key present but null — nullable, so still valid
+        $this->assertTrue($this->model->execute(['firstname' => 'John', 'telephone' => null]));
+    }
+
+    /**
+     * Address with a NonNull field present and filled passes validation.
+     */
+    public function testAddressWithAllNonNullFieldsPresentPassesValidation(): void
+    {
+        $firstnameField = $this->makeField('firstname', true);
+        $telephoneField = $this->makeField('telephone', false);
+
+        $this->cartAddressInputType->method('getFields')
+            ->willReturn(['firstname' => $firstnameField, 'telephone' => $telephoneField]);
+
+        $this->assertTrue($this->model->execute([
+            'firstname' => 'John',
+            'telephone' => '+15005550006',
+        ]));
+    }
+
+    /**
+     * Address with a NonNull field key present but set to null fails validation.
+     */
+    public function testAddressWithNonNullFieldSetToNullFailsValidation(): void
+    {
+        $firstnameField = $this->makeField('firstname', true);
+
+        $this->cartAddressInputType->method('getFields')
+            ->willReturn(['firstname' => $firstnameField]);
+
+        // Key exists, value is null → invalid for NonNull field
+        $this->assertFalse($this->model->execute(['firstname' => null]));
+    }
+
+    /**
+     * Address missing a NonNull field key entirely still passes (output validator, not input).
+     *
+     * ValidateAddressFromSchema is used on the output side to decide whether a saved address
+     * is complete enough to return. A key simply not being present is allowed; only an
+     * explicitly-null value for a NonNull field is rejected.
+     */
+    public function testAddressWithMissingNonNullFieldKeyPassesValidation(): void
+    {
+        $firstnameField = $this->makeField('firstname', true);
+
+        $this->cartAddressInputType->method('getFields')
+            ->willReturn(['firstname' => $firstnameField]);
+
+        // Key does not exist at all → validator treats as acceptable (saved address may be partial)
+        $this->assertTrue($this->model->execute([]));
+    }
+
+    /**
+     * Build a mock InputObjectField with a name and a nullable/NonNull type.
+     */
+    private function makeField(string $name, bool $isNonNull): InputObjectField
+    {
+        $field = $this->getMockBuilder(InputObjectField::class)
+            ->disableOriginalConstructor()
+            ->onlyMethods(['getType'])
+            ->getMock();
+        $field->name = $name;
+
+        if ($isNonNull) {
+            $type = $this->getMockBuilder(NonNull::class)
+                ->disableOriginalConstructor()
+                ->getMock();
+        } else {
+            $type = $this->createMock(StringType::class);
+        }
+
+        $field->method('getType')->willReturn($type);
+
+        return $field;
+    }
+}
diff --git a/app/code/Magento/QuoteGraphQl/etc/schema.graphqls b/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
index 1ace0a4332f5..5e584c99f976 100644
--- a/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
+++ b/app/code/Magento/QuoteGraphQl/etc/schema.graphqls
@@ -173,7 +173,7 @@ input CartAddressInput @doc(description: "Defines the billing or shipping addres
     region_id: Int @doc(description: "An integer that defines the state or province of the billing or shipping address.")
     postcode: String @doc(description: "The ZIP or postal code of the billing or shipping address.")
     country_code: String! @doc(description: "The country code and label for the billing or shipping address.")
-    telephone: String! @doc(description: "The telephone number for the billing or shipping address.")
+    telephone: String @doc(description: "The telephone number for the billing or shipping address.")
     vat_id: String @doc(description: "The VAT company number for billing or shipping address.")
     save_in_address_book: Boolean @doc(description: "Determines whether to save the address in the customer's address book. The default value is true.")
     fax: String @doc(description: "The customer's fax number.")